Low power reentrant stream crossed-field noise generator tube

ABSTRACT

A crossed-field reentrant stream noise generator tube is disclosed. The tube is of the circular crossed-field geometry having a cathode electrode structure coaxially disposed of an anode electrode structure to define an unobstructed crossed-field interaction region in the space between the anode and cathode such that the electron stream can circulate completely around the cathode. The cathode electrode structure includes one or more thermionic emitter buttons forming the emitting portion and the remaining portion of the cathode forms a nonemitting sole structure. A microwave circuit is formed in the anode structure facing an arcuate portion of the cathode structure. In one embodiment, the nonemitting sole portion of the cathode structure is treated to inhibit secondary emission therefrom by reducing the secondary emission ratio of the surface to less than 1. In a second embodiment, the nonemitting sole portion of the cathode structure is biased negatively relative to the thermionic emitter to prevent bombardment of the sole and thus to inhibit secondary emission therefrom.

United States Patent [72] Inventor Hunter L. McDowell Chatham, NJ. [21]Appl. No. 845,000 [22] Filed July 25, 1969 [45] Patented Sept. 28, 1971[73] Assignee Varian Associates Palo Alto, Calif.

[54] LOW POWER REENTRANT STREAM CROSSED- FIELD NOISE GENERATOR TUBE 5Claims, 5 Drawing Figs.

[52] U.S. Cl 331/78, 313/106, 315/393 [51 Int. Cl ..H01j 25/34, H03b29/00 [50] Field ofSearch 331/78; 315/35, 39.3; 313/106, 107

[56] References Cited UNITED STATES PATENTS 2,942,140 6/1960 Guilbaud315/393 X 2,992,360 7/1961 Reverdin. 315/393 3,192,434 6/1965 Hull315/393 Primary Examiner- Roy Lake Assistant Examiner- Palmer C. DemeoAttorneys-Stanley Z. Cole and Gerald L. Moore ABSTRACT: A crossed-fieldreentrant stream noise generator v. tube is disclosed. The tube is ofthe circular crossed-field geometry having a cathode electrode structurecoaxially disposed of an anode electrode structure to define anunobstructed crossed-field interaction region in the space between theanode and cathode such that the electron stream can circulate completelyaround the cathode. The cathode electrode structure includes one or morethermionic emitter buttons forming the emitting portion and theremaining portion of the cathode forms a nonemitting sole structure. Amicrowave circuit is formed in the anode structure facing an arcuateportion of the cathode structure. In one embodiment, the nonemittingsole portion of the cathode structure is treated to inhibit secondaryemission therefrom by reducing the secondary emission ratio of thesurface to less than 1. In a second embodiment, the nonemitting soleportion of the cathode structure is biased negatively relative to thethermionic emitter to prevent bombardment of the sole and thus toinhibit secondary emission therefrom.

PATENTEDSEP2819?! I I S 09 I 81 FIG. I; PRIOR ART, e

INVENTOR. HUNTER L. MCDOWELL ATTORNEY LOW POWER REENTRANT STREAMCROSSED-FIELD NOISE GENERATOR TUBE DESCRIPTION OF THE PRIOR ARTHeretofore, crossed-field reentrant stream noise generator tubes havebeen constructed. In these tubes, a single thermionic emitter has beenemployed which has been connected for operation at the same potential asthe remainder of the cathode structure. In such a tube, back bombardmentof the cathode sole portion by electrons driven out of the electronstream by r.f. energy produces secondary emission which reinforces theelectron system as it recirculates about the cathode in the reentrantinteraction region. While such tubes are generally useful for producingwide band noise outputs at relatively high-power levels, it has beenfound that when an attempt is made to operate these tubes in arelatively low-power regime, as of in the neighborhood of 10 watts CW,that the efficiency of the tube becomes so low that its usefulness isgreatly impaired.

SUMMARY OF THE INVENTION The principal object of the present inventionis the provision of an improved low-power reentrant stream crossed-fieldnoise generator tube.

One feature of the present invention is the provision of a cathodestructure including one or more thermionic emitters and a cold cathodesole portion with means for inhibiting secondary emission from the soleportion such that the noise generator tube may be operated at relativelylow-power levels.

Another feature of the present invention is the same as the precedingfeature wherein the secondary emission from the sole portion of thecathode is inhibited by insulating the sole relative to the thermionicemitter and operating the sole portion at a negative potential relativeto the thermionic emitter, whereby back bombardment of the cathode soleby electrons driven out of the stream is substantially reduced toinhibit secondary emission from the sole.

Another feature of the present invention is the same as the firstfeature wherein secondary emission from the sole portion of the cathodeis inhibited by treating the surface of the sole to reduce the effectivesecondary emission ratio of its surface to less than 1.

Another feature of the present invention is the same as the precedingfeature wherein the cathode sole is coated with a nonsecondary emissivecoating or is grooved for trapping secondary emission, thus reducing itseffective secondary emission ratio.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of thepresent invention will become apparent upon perusal of the followingspecification taken in connection with the accompanying drawingswherein:

FIG. I is a schematic line diagram of a prior art noise generator tube,

FIG. 2 is a schematic line diagram of a crossed-field noise generatortube incorporating features of the present invention,

FIGS. 3 and 4 are enlarged detail views of alternative embodiments of aportion of the sole structure of FIG. 2 delineated by lines 33 and 4-4,and

FIG. 5 is a schematic line diagram of an alternative embodiment of anoise generator tube incorporating features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 thereis shown a typical prior art crossed-field reentrant stream noisegenerator tube 1.

Tubes of this general type are disclosed in U.S. Pat. application557,398 filed June 14, 1966, now U.S. Pat. No. 3,437,866, and assignedto the same assignee as the present invention. Briefly, the tube 1includes a cathode electrode structure 2 coaxially surrounded by ananode structure 3 to define an annular reentrant electron streamcrossed-field interaction region 4 in the annular space between thecathode 2 and the anode 3.

The anode structure 3 includes a slow wave microwave circuit 5 disposedfacing the cathode 2. The anode structure 3 also includes a circuitsever portion 6 defining an upstream terminal end 7 and a downstreamoutput terminal end 8 of the microwave circuit 5 adjacent the oppositeends of the circuit sever 6. A distributed attenuator structure 9 isdisposed adjacent the upstream end of the microwave circuit 5 forterminating the upstream end of the microwave circuit 5 to prevent wavereflections on the circuit which might otherwise lead to undesiredcoherent oscillations of the tube. A coaxial line 11 is coupled to theoutput terminal end 8 of the microwave circuit 5 for coupling microwavenoise energy generated on the microwave circuit 5 to a suitable load,not shown. A magnet, not shown, produces a static axially directedmagnetic field H in the interaction region 4.

The cathode structure 2 includes a thermionic priming electron emitterbutton 12 disposed adjacent the circuit sever 6 for injecting a streamof electrons into the annular interaction region 4 to initiate operationof the tube. The priming electron emitter 12 is relatively small andelectrically connected to a remaining sole portion 13 of the cathodeelectrode 2. The sole portion 13 serves as a secondary electron emitterwhen bombardment by electrons driven by r.f. energy from the electronstream to produce a copious supply of secondary electrons to replenishthe electrons of the electron stream circulating about the cathodestructure 2 in a reentrant manner. The reentrant nature of the electronstream enhances efficiency of the tube since the electron in the streamare debunched in the r.f. field free region adjacent the circuit severportion 6 and can contribute to the electron stream current supplied bythe thermionic emitter 12.

The noise generator tube 1 of FIG. I operates relatively efficiently athigh-output power levels and is capable of delivering a relatively wideband of noise output. However, when the tube of FIG. I is attempted tobe operated at relatively lowpower levels, as of in the range of 1-l0watts CW, the efficiency of the tube drops to a small fraction of 1percent. It is believed that the relatively poor performance of theprior art tube at low-power levels is due to the fact that a largecirculating electron stream is generated by secondary electron emissionand the noise energy content of the electron stream is distributed overan extremely wide spectrum. At low-power levels the r.f. fieldassociated with the noise power on the slow wave circuit and within thepass band of the microwave circuit 5 is extremely low and is not capableof controlling the space charge so as to concentrate the noise energywithin the circuit bandwidth, as is the case in higher power noisegenerators.

Referring now to FIG. 2 there is shown a microwave noise generator tube15 incorporating features of the present invention. The tube 15 of FIG.2 is substantially identical to that of FIG. 1 with the exception thatthe thermionic cathode-emitter button 12 has been movedcircumferentially to a position axially coextensive with a portion ofthe microwave anode circuit 5 and is disposed facing the circuit 5. Inaddition, the remaining sole portion 13 of the cathode 2 has its surfacetreated so as to reduce its secondary emission ratio to less than I. Inthis manner, the reentrant electron stream is principally produced byprimary thermionic emission such that the noise power content thereof isconcentrated within a narrower band of frequencies, some of which willbe within the pass band of the microwave circuit 5. In one embodiment ofthe cathode sole structure, as illustrated in FIG. 3, the surface of thesole electrode 13 which faces the interaction space 4 is coated orformed of a material having a secondary emission ratio less than 1.Suitable coatings or materials include titanium, carbon and tungstencarbide.

As an alternative to forming the surface of the sole 13 of a materialhaving a secondary emission ratio less than 1, the surface of thecathode sole 13 which faces the interaction region 4 may be grooved withaxially directed grooves 18, as shown in FIG. 4. The width w of thegrooves 18 is preferably wider than the land portions 17 remainingbetween the grooves. The depth d of the grooves is preferably greaterthan the width of the grooves such that secondary electrons liberatedwithin the grooves are captured by an adjacent land 17. In this'manner,the secondary electrons are trapped and the effective secondary emissionratio of the grooved surface is reduced to less than i. By inhibitingsecondary emission from the sole 13 and relying principally upon thethermionic emission from he emitter buttons 12 the noise tube 15 isoperable within the low-power regime of between 1 and watts with low butacceptable efficiencies. Efficiencies of up to 5 percent have beenachieved with less than optimized conditions.

Referring now to FIG. 5 there is shown an alternative noise tubeembodiment of the present invention. More particularly, there is shown amicrowave noise generator tube 21 which is essentially the same as thatof FIG. 2 with the exception that the sole portions of the cathodestructure 2 are insulated from and biased negative relative to thepotential applied to the thermionic emitter buttons 12 via insulators 22such that the recirculating electron stream is moved closer to the anodecircuit 5 to substantially reduce back bombardment of the cathode soleportions 13. In this manner, secondary emission from the sole portions13 is substantially inhibited.

in addition, a plurality of primary thermionic emitter buttons 12 arepreferably provided at intervals circumferentially spaced about theperiphery of the cathode electrode structure 2 with intervening soleportions 13. In this manner, as the electron stream is collected on themicrowave circuit 5, in the downstream regions of the circuit, due tothe increased interaction between the growing r.f. microwave energy onthe circuit and the electron stream, the stream is replenished from theadditional thermionic emitter or emitters 12. This improves the gain andefficiency of the tube.

In a typical example of a tube 21 as shown in FIG. 5, the microwavecircuit 5 is operated at ground potential, the sole portions 13 areoperated at 2,000 volt potential, and the thermionic cathode emitters 12are operated at l,400 volt potential Noise tube embodiments of FIGS. 2and 5 may each provided with two or more thermionic emitters 12 disposedabout the periphery of the cathode electrode 2.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is 1. In a crossed-field reentrant stream noisegenerator tube, means forming a cathode electrode structure, meansforming an anode electrode structure having a microwave circuit portiongenerally concentrically disposed of said cathode structure to define acrossed-field reentrant stream interaction region in an unobstructedannular space between said cathode and anode structures, means forming aplurality of discrete thermionic emitters circumferentially spaced apartaround the periphery of said cathode structure to inject electrons intosaid interaction region for interaction with microwave energy on saidmicrowave circuit portion to produce output microwave noise energy, saidanode electrode structure including a circuit sever portion defining anupstream end and an output downstream end for said microwave circuitportion adjacent the ends of said circuit sever portion, means forming aresistive termination for terminating the upstream end of said microwavecircuit portion, THE IMPROVEMENT WHEREIN, said cathode electrodestructure includes cathode sole portions disposed facing-said microwavecircuit portion and separating said thermionic emitters, each of saidthermionic emitters being positioned on said cathode structure oppositesaid microwave circuit portion, and means for inhibiting secondaryemission from said cathode sole portions of said cathode structure,whereby the noise generator tube may be operated at relatively low-powerlevels.

2. The apparatus of claim 1 wherein said means for inhibiting secondaryelectron emission from said cathode sole portions includes an electricalinsulator structure disposed between each of said thermionic cathodeemitters and said cathode sole portions for holding off a negative biaspotential applied to said cathode sole portions relative to thepotential applied to said thermionic emitters, whereby back bombardmentof said cathode sole portions by electrons driven out of the electronstream is substantially reduced in use to inhibit secondary emissionfrom said sole portions of said cathode structure.

3. The apparatus of claim 1 wherein the surfaces of said sole portionsof said cathode structure which face the interaction region are formedin such a way as to reduce the effective secondary emission ratio of thesurfaces to less than I 4. The apparatus of claim 1 wherein the surfacesof said sole portions of said cathode structure which face theinteraction region are grooved to reduce the effective secondaryemission ratio of the surfaces by trapping secondary electrons in thegrooves in the surfaces of said sole portions.

5. The apparatus of claim 1 wherein the surfaces of said sole portionswhich face the interaction region are formed of a material having asecondary emission ratio less than 1.

1. In a crossed-field reentrant stream noise generator tube, meansforming a cathode electrode structure, means forming an anode electrodestructure having a microwave circuit portion generally concentricallydisposed of said cathode structure to define a crossed-field reentrantstream interaction region in an unobstructed annular space between saidcathode and anode structures, means forming a plurality of discretethermionic emitters circumferentially spaced apart around the peripheryof said cathode structure to inject electrons into said interactionregion for interaction with microwave energy on said microwave circuitportion to produce output microwave noise energy, said anode electrodestructure including a circuit sever portion defining an upstream end andan output downstream end for said microwave circuit portion adjacent theends of said circuit sever portion, means forming a resistivetermination for terminating the upstream end of said microwave circuitportion, THE IMPROVEMENT WHEREIN, said cathode electrode structureincludes cathode sole portions disposed facing said microwave circuitportion and separating said thermionic emitters, each of said thermionicemitters being positioned on said cathode structure opposite saidmicrowave circuit portion, and means for inhibiting secondary emissionfrom said cathode sole portions of said cathode structure, whereby thenoise generator tube may be operated at relatively low-power levels. 2.The apparatus of claim 1 wherein said means for inhibiting secondaryelectron emission from said cathode sole portions includes an electricalinsulator structure disposed between each of said thermionic cathodeemitters and said cathode sole portions for holding off a negative biaspotential applied to said cathode sole portions relative to thepotential applied to said thermionic emitters, whereby back bombardmentof said cathode sole portions by electrons driven out of the electronstream is substantially reduced in use to inhibit secondary emissionfrom said sole portions of said cathode structure.
 3. The apparatus ofclaim 1 wherein the surfaces of said sole portions of said cathodestructure which face the interaction region are formed in such a way asto reduce the effective secondary emission ratio of the surfaces to lessthan 1 .
 4. The apparatus of claim 1 wherein the surfaces of said soleportions of said cathode structure which face the interaction region aregrooved to reduce the effective secondary emission ratio of the surfacesby trapping secondary electrons in the grooves in the surfaces of saidsole portions.
 5. The apparatus of claim 1 wherein the surfaces of saidsole portions which face the interaction region are formed of a materialhaving a secondary emission ratio less than 1.